The present invention relates to an image recording method by means of ink thermal transfer, and particularly to a thermal transfer recording method and apparatus which can give a preferred halftone image by means of ink thermal transfer of both sublimation type and fusion type.
Up to the present there have been developed thermal transfer recording apparatus of sublimation type and thermal transfer recording apparatus of fusion type, and they have been used properly according to their intended purpose.
A thermal transfer recording apparatus of sublimation type can change the density level of each pixel, and in case of color printing, it can express n.sup.3 colors for each pixel when recording each pixel in one of n density levels for each color by means of 3-color ink sheets of cyan, magenta and yellow. In case of recording each pixel in 256 density levels for each color, for example, about 16.7 million colors can be reproduced. Since this type can provide beautiful printing, therefore, this type is often used for image printing. But this type has disadvantages of long printing time, high running cost, and the like.
On the other hand, a thermal transfer recording apparatus of fusion type uses a density pattern method, systematic dither method and the like to obtain a gradated image, since this type expresses each pixel in 2-level (on/off) signals for 3 colors of cyan, magenta and yellow for example, in the case of color printing. The density pattern method obtains pseudogradation of images with variation of pixel area by expressing each pixel with a dot matrix of 2.times.2 dots, 4.times.4 dots, or the like, while the systematic dither method, which expresses one pixel with one dot, obtains pseudogradation by arranging each dot in a dither matrix. Therefore, the fusion type is inferior to the sublimation type in expressing gradation, but has advantages that it is shorter in printing time and less expensive in running cost by 1/3 to 1/5 in comparison with the sublimation type.
FIG. 3 is a partial explanatory drawing of heat generating resistive members of a thermal head mounted on a known ink thermal transfer image recording apparatus of sublimation type.
In FIG. 3, a glaze layer (not shown in FIG. 3) of glass is formed on an insulating substrate (not shown in FIG. 3) of alumina or the like, a heat generating resistive film of Ta.sub.2 N, Ta-SiO.sub.2 or the like is formed on the glaze layer by means of vapor deposition, sputtering or the like, and this resistive film is formed into a plurality of rectangular members 3, composed of members 3a and 3b, aligned in a straight line by means of photoetching. A common electrode 1 is formed commonly to the respective heat generating resistive members 3 on the upper face of them, and individual electrodes 2 are formed respectively on the lower faces of the heat generating resistive members. The other ends of the common electrode 1 and the individual electrodes 2 are respectively connected with terminals of a driving IC (not shown in FIG. 3), and the driving IC outputs signals for individually and selectively driving a plurality of heat generating resistive members 3. Each heat generating resistive member 3 is composed of heat generating resistive members 3a and 3b which are insulated from each other by a respective slit 4 of insulating material and adjacent heat generating resistive members 3 are insulated from each other by an insulating member 5.
The thermal head has an antioxidant film layer and wear resistant layer not shown in the figure which are formed on the common electrode 1, individual electrodes 2 and the heat generating resistive members 3. Each individual heat generating resistive member 3 forms independently a heat generating element corresponding to one dot of a minimum printing unit. The heat generating element generates heat by applying voltage between the common electrode 1 and associated individual electrode 2.
Such thermal transfer recording apparatus can make a desired printing by pressing the thermal head composed of the heat generating resistive members 3 aligned in a line in a horizontal scanning direction against a sheet of recording paper through an ink sheet, applying a specified signal to an individual electrode 2 corresponding to a desired dot on the basis of specified printing information to make the heat generating resistive member 3 connected with the individual electrode 2 generate heat, and transferring ink from the ink sheet to the recording paper.
FIGS. 4(a)-(c) are explanatory drawings of patterns appearing on a recording paper when recording by means of an ink thermal transfer method of sublimation type, and the patterns are obtained by the thermal head shown in FIG. 3. The thermal head shown in FIG. 3 is made so that a printed dot shape may be longer in the vertical scanning direction, and the relation between M representing dot length in the horizontal scanning direction and S representing dot length in the vertical scanning direction is set as S/M&gt;1. The reason is to prevent occurrence of a hollow of density between printed lines.
FIG. 5 shows the distribution of temperature on a heat generating resistive member 3 of the thermal head shown in FIG. 3. Temperature distribution on one pixel is made flat and sublimative dye is uniformly distributed by dividing the heat generating resistive member 3 into two parts by the slit 4. However, if making the respective dot lengths in the horizontal and vertical scanning directions equal to a desired pixel size as shown by the heat generating resistive members 7 insulatingly divided by a slit 9, as shown in FIG. 7(a), temperature distribution in the horizontal scanning direction is made nearly uniform over a pixel area as shown in FIG. 7(b). However, as for temperature distribution in the vertical direction, shown in FIG. 7(c), the difference in temperature between the middle part and the peripheral parts of the heat generating resistive member 7 is increased in a pixel area. As a result, occurrence of a hollow of density between printed lines creates a discontinuity in density when printing an image to be uniform in density so as to significantly deteriorate image quality.
In order to solve this problem, the relation between M representing dot length in the horizontal scanning direction and S representing dot length in the vertical scanning direction is set as S/M&gt;1, and the temperature distribution in this case is as shown in FIGS. 6(a)-6(c). If the dot length in the vertical scanning direction is made longer than the desired pixel size (the middle part in the FIG. 6(a)) as shown by the heat generating resistive members 6 insulatingly divided by a slit 8 in FIG. 6(a), the temperature distribution in the horizontal scanning direction shown in FIG. 6(b) is made nearly uniform in a pixel area similarly to FIG. 7(b), and for temperature distribution in the vertical scanning direction also, the difference in temperature between the middle part and peripheral part of the heat generating resistive member 6 is decreased as shown in FIG. 6(c).
FIG. 8 is a partial explanatory drawing of heat generating resistive members of a thermal head of heat concentration type mounted on a known ink thermal transfer image recording apparatus of fusion type. The thermal head is composed of insulating parts 10 each of which is formed as a circular hole, heat generating resistive members 11 separated by insulating strips, a common electrode 12 and individual electrodes 13.
FIG. 9(a)-9(c) is an explanatory drawing of patterns appearing on a recording paper when recording by mean of an ink thermal transfer method of fusion type, and the patterns are obtained by the thermal head shown in FIG. 8. The thermal head shown in FIG. 8 makes 4 dots in one pixel area with variation of current density in the heat generating resistive member 11 caused by the insulating parts 10 formed as circular holes. As the power applied to an individual electrode 13 increases, the density level of the pixel increases in the manner of an area density level system in the order shown successively in FIGS. 9(a), (b) and (c).
Known thermal transfer recording apparatus of both sublimation type and fusion type whose proper dimensions of heat elements of the thermal head for beautiful printing are different between the sublimation type and fusion type can not be properly used as both types and are composed in combination of thermal transfer recording devices of a simple sublimation type and fusion type in fact.
In other words, since M representing the length of a heat generating element in the horizontal scanning direction and S representing the length of the element in the vertical scanning direction are set as relatively S/M&gt;1 to S/M.apprxeq.2 in the heat generating resistive members of the thermal head used for sublimation type thermal transfer recording, if the same thermal head is used for fusion type thermal transfer recording, ink of an area equivalent to the heat generating element is transferred from the ink sheet to a recording medium to make the dot length too long in the vertical scanning direction, and as a result, line widths become different in the horizontal and vertical scanning directions and deterioration caused by battered dots of pseudogradation made by means of a systematic dither method or the like significantly deteriorates the printing quality. On the other hand, since M representing the length of a heat generating element in the horizontal scanning direction and S representing the length of the element in the vertical scanning direction are set as relatively S/M.apprxeq.1 in the heat generating resistive members of the thermal head used for fusion type thermal transfer recording, if the same thermal head is used for sublimation type thermal transfer recording, occurrence of a hollow of density between printed lines cause discontinuous density when printing an image which is to be uniform in density and significantly deteriorates the image quality.